1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a compound semiconductor light emitting device for obtaining light from a compound semiconductor layer, and more particularly, to a compound semiconductor light emitting device which uses a transparent conductive film as a window electrode.
2. Description of the Related Art
Gallium nitride compound semiconductors, such as GaN, AlGaN, InGaN, and InGaAlN, are drawing attention as visible light emitting devices of green, blue, or the like.
In the manufacture of optical devices using these gallium nitride compound semiconductors, since there are a small number of substrates which have a lattice match with a gallium nitride compound semiconductor, sapphire is normally used as a substrate for crystal growth. Moreover, when an insulative substrate such as sapphire is used, unlike other light emitting devices using a conductive semiconductor substrate such as GaAs and InP, an electrode cannot be provided on the substrate side. Therefore, the p-side electrode and the n-side electrode to be provided on the semiconductor layer have to be formed on one surface side of the substrate on which the semiconductor layer is laminated.
Therefore, in order to suppress the reduction in the light transmissivity of emitting light, there is proposed a gallium nitride compound semiconductor light emitting device provided with a translucent electrode (for example, refer to Patent Document 1 below).
FIG. 1 is a cross-section showing an example of a related art gallium nitride compound semiconductor light emitting device provided with a translucent electrode. In the gallium nitride compound semiconductor light emitting device 300, on one surface (upper surface in FIG. 1) of a sapphire substrate 301 is provided an n-type GaN layer 305 via a GaN buffer layer 303, and on one surface (upper surface in FIG. 1) of this n-type GaN layer 305 are provided a p-type GaN layer 307 containing Mg as a p-type dopant, and an n-side electrode 309 formed from Ti/Au and the like. The periphery of the n-side electrode 309 is enclosed with an SiO2 film 311a, so that it is electrically insulated from the p-type GaN layer 307.
On the other hand, on the p-type GaN layer 307 are provided an SiO2 film 311b and a metal thin film layer 313 containing Mg. Furthermore, on this metal thin film layer 313 is provided a transparent conductive film 315 having a thickness of 100 nm, formed from an indium tin oxide (ITO) film for current diffusion. A p-side electrode 317 formed from Ti/Au and the like, is provided so as to cover the SiO2 film 311b and a part of the transparent conductive film 315.
That is, in this structure, light emitting from a junction interface between the n-type GaN layer 305 and the p-type GaN layer 307 can be taken out through the transparent conductive film 315. In FIG. 1, the dotted lines show an electrical current flowing from the p-side electrode 317 through transparent conductive film 315 toward the junction interface. On the other hand, the alternate long and short dash lines show a state where light emitting from the junction interface mainly passes through the transparent conductive film 315 and radiates to the outside.
The transparent conductive film 315 formed from the indium tin oxide (ITO) film contains Sn as an n-type impurity, and thus generally is unable to be formed on the p-type GaN layer 317. Therefore, the metal thin film layer 313 containing Mg which is relatively easy to bring into ohmic contact with the p-type GaN layer 317, is formed in a thickness of 2 nm which is a thickness having 70% transmissivity with respect to the emitting light. Furthermore, in order to reduce the sheet resistance of the metal thin film layer 313, the transparent conductive film 315 having a thickness of 100 nm is formed.
After forming the transparent conductive film 315 on the deposited film of Mg/Ni=1 nm/2 nm, by performing annealing at 500° C. for 10 minutes, the adhesiveness and the ohmic contact between the metal thin film layer 313 and the p-type GaN layer 307 can be achieved. Since the metal thin film layer 43 contains Mg, if annealing is performed as it is, the metal thin film layer 313 is evaporated to vanish or to become thinner. However, since the transparent conductive film 315 provided on the metal thin film layer 313 acts as a protective film, the metal thin film layer 313 is prevented from being evaporated or vanishing. As a result, a reduction in the controllability of the metal film thickness on the electrode process, and the accompanying deterioration of the I-V characteristics of the light emitting device are avoided.
Moreover, a gallium nitride compound semiconductor light emitting device is proposed in which a transparent conductive film is provided as an electric current diffusion layer on a p-type GaN layer doped with Mg as a p-type dopant, by forming a first indium tin oxide (ITO) film by an evaporation method, and thereon a second indium tin oxide (ITO) film by a sputtering method (for example, refer to Patent Document 2 below).
FIG. 2 is a cross-section showing a gallium nitride compound semiconductor light emitting device 400 formed with such double layers of indium tin oxide. On one surface of the sapphire substrate 410 are sequentially provided: an AlN buffer layer 420; an Si-doped GaN layer 430 having a thickness of about 1.2 μm; an active layer 440 having a thickness of about 40 nm comprising a multiple quantum well (MQW) of n-type GaN and n-type InGaN; a cap layer 450 having a thickness of about 20 nm comprising a superlattice of AlN and p-type GaN; an Mg-doped GaN layer 460 having a thickness of about 200 nm; and a Zn film 470 having a thickness of several nm. A transparent conductive film 480 is provided thereon by forming: a lower indium tin oxide film 481 having a thickness of about 10 nm serving as a lower transparent conductive film; and an upper indium tin oxide film 482 having a thickness of about 500 nm serving as an upper transparent conductive film on this lower indium tin oxide film 481. Furthermore, on the partially exposed Si-doped GaN layer 430 is provided an n-type electrode 491, and on the upper indium tin oxide film 482 is provided a p-type electrode 492.
It is described that, since the lower indium tin oxide film 481 is formed by the evaporation method and the upper indium tin oxide film 482 is formed by the sputtering method, a compound semiconductor light emitting device 400 can be obtained in which the Mg-doped GaN layer 460 is less damaged, the operating voltage is lower, and the take-out efficiency of light to the outside is higher, in this case where the lower indium tin oxide film 481 is formed by the evaporation method, compared to the case where the transparent conductive film 480 is formed only by the sputtering method.
Furthermore, rather than a transparent conductive film formed from an indium tin oxide, there is proposed a gallium nitride compound semiconductor light emitting device provided with a translucent electrode formed from a metal (for example, refer to Patent Document 3 below).
As an example of such a device, FIG. 3 is a cross-section showing a gallium nitride compound semiconductor light emitting device 500 formed with a translucent electrode 505 by means of deposition of Ni and Au. On one surface of the sapphire substrate 501 are sequentially formed an n-type GaN layer 502 and a p-type GaN layer 503. On the p-type GaN layer is provided a metal translucent electrode 505 formed from Ni/Au. It is described that, the translucent electrode 505 is obtained by depositing Ni to a thickness of 30 nm on the p-type GaN layer 503, and depositing Au to a thickness of 70 nm on the Ni in a deposition apparatus, and after the deposition, annealing the resultant product at 500° C. for 10 minutes in an annealing apparatus for alloying and providing a transmissive property, so that a light emitting diode having satisfactory luminous efficiency and production yield can be produced.
However, in the above gallium nitride compound semiconductor light emitting devices 300, 400, and 500, the refractive index of the transparent conductors 315 and 480 and the translucent electrode 505 is about 2.0, and the refractive index of an air space is about 1.0, thus causing a problem in that, when light emitted in the light emitting layer passes through the transparent conductor or the translucent electrode and emits into the air space, the total reflectivity in the interface (light emitting surface) between the transparent conductor or the translucent electrode and the air space is increased and the intensity of light varies according to the direction.
The following documents describe the related art for the current invention:
Patent Document 1: Japanese Patent Publication No. 3207773
Patent Document 2: Japanese Patent Publication No. 3394488
Patent Document 3: Japanese Patent Publication No. 2803742